Reference signals such as Sounding Reference Symbols (SRS) are transmitted on the uplink by for example a wireless terminal (WT) so that a network node such as eNodeB (eNB) can estimate the signal strength or quality at different antennas of the network node with relation to the reference signal. The estimate can be used to do receiving point selection in uplink (UL) multi-sector cell, where a receiving point refers to a sector that is equipped with a number of RX antennas.
Multi-sector cell, also called merged cell or shared cell in some cases, is a new cell configuration for LTE and enables a multi Radio Remote Unit (RRU) deployment without needing to care about cell planning from a Radio Frequency (RF) perspective. It is achieved by allowing the different RRUs using the same Physical Cell Identity (PCI) and thus all RRUs are considered, by the WT, to be part of the same cell. The spatially separated RRU or a group of RRUs are called sector. A cell can contain multiple sectors, and a WT can belong to one sector or multiple sectors depending on the WT position.
Different from a basic LTE cell configuration where all WTs camped in that cell shall share cell resources by time and/or frequency multiplexing, in a multi-sector cell, yet another resource domain, spatial resource, is also introduced.
Some WTs that are spatially separated can use the same time and frequency resource, but on different sectors. On the other hand, where a WT's transmission can be detected in several sectors, receiving point selection is used in order to obtain macro diversity gain. To be able to determine if several WTs can use the same time and frequency resource, or if receiving point selection is to be used, the eNB must has the knowledge of signal quality of all WTs that are received by every sector, which can be obtained from sounding reference signals SRS. By computing the difference of the received signal power from SRSs, the isolation degree among different sectors is defined. The degree of isolation is further used as criteria to determine if several WTs are interfered with each other or if they are co-scheduled in the same time and frequency resource but in different sectors.
Different WTs can be assigned with a separate SRS resource in the same cell. The separation can be achieved by for example any one or more of, time division multiplexing (TDM), frequency division multiplexing (FDM), and code division multiplexing (CDM). If the WTs are separated by TDM or FDM, they will not cause any interference to each other. If CDM is used, the interference from other WTs is inevitable. In the case of multi-cell scenario, interference may also come from neighboring cell, and thanks to SRS signal design, this neighboring cell's SRS can be viewed as white noise.
Conventional technique for measurement in SRS is based on channel estimation. Channel estimation usually includes a matched filtering, Inverse Fast Fourier Transform (IFFT), noise suppression, and Fast Fourier Transform (FFT), which is computationally intensive. Furthermore, the amount of Digital Signal Processing (DSP) resources that can be allocated to SRS is limited due to the additional support for the Physical Uplink Shared CHannel (PUSCH) and Physical Uplink Control CHannel PUCCH. The problem with the computational intense channel estimation becomes even worse in a multi-sector scenario, where a signal from a WT is received by antennas from all sectors and the channel estimation needs to be done on each receive antenna. The problem becomes even more severe as the system capacity is limited dramatically when there is a large number of sectors configured with the same cell ID. One example is the requirement of combining 144 sectors into one cell
In J. Chang et al, “Apparatus and method for estimating interference and noise in a communication system”, November 2009, a generic method for OFDM applications that estimates the signal power and interference plus noise power was proposed by assuming similar channel characteristics between adjacent sub-carriers in order to reduce the computational complexity. This method can be applied to SRS measurement. However, the method only deals with one signal source and not signals from multiple interfering sources.